The phase behaviour of the binary copolymer systems SMA-SAN and SAN-SAN can be described well using the copolymer repulsion model. However, the phase behaviour of the ternary system consisting of SMA-SAN(1)-SAN(2) with SAN(1) and SAN(2) having different chemical compositions is not known. In order to reveal this, the composition of the blends, the composition of the SMA (ranging from 22 to 34 wt% MA) at constant SAN compositions (26, 34 and 28, 32 wt% AN), the molar mass of the SMA (M-W = 3.5 and 110 kg/mol) as well as the temperature (25 and 230 degreesC) were varied. The number of phases was studied using differential scanning calorimetry (DSC) and a critical evaluation of the predicted and observed glass transition temperatures based on the Fox equation. The binodals and spinodals were calculated using the Flory-Huggins approach. From a comparison of predicted and observed miscibility, it is concluded that this ternary system can be described using the aforementioned model.